On Sun, Jun 11, 2017 at 10:18 PM, tim prince via gcc-help <gcc-help@xxxxxxxxxxx> wrote: >> Is there a way to find out the maximum alignment supported by the iinker? > > You may need to look at the binutils source (if your gcc uses gnu ld), > where the maximum supported alignment is defined. Not documented as such - source code time. That's okay. > 32-byte alignment needs > to be supported for x86-64 platforms beginning with Intel Nehalem; 64-byte > alignment for those which support AVX512. So it is a poor quality build of > binutils which doesn't meet those requirements. Not many years ago, the > binutils default on windows was less than 16-byte alignment and so it was > useful to build your own copy with this corrected, but there may be a limit > to what the platform can support. Surely, you can run objdump on your > executables and see what alignments you get in those spots where you set a > request. Agreed that 128-byte alignment on x86 might avoid some situations > where the prefetcher brings in a useless adjacent cache line, but the cache > line size is 64 bytes. I read just a few days ago that Intel x86 (not x86_64) cache line lengths varied by processor from 32 to 128 bytes. AFAIK, x86_64 is always 64 bytes, but I have no strong confirmation of that. I have read Intel these days in hardware (sometimes something you can switch off in the BIOS) always brings over two cache lines at a time, so if you're trying to isolate a variable from other activity you have to treat the cache linn size as if it's 128 bytes. Thanks for your help, Tim!
